An alloy is generally a mixture of metal elements, or a mixture of metal and other nonmetal elements. Examples of alloys include steel, solder, brass, etc. made by mixing two or more elements in a molten state to form a molten mixture. When the molten mixture cools, the alloy precipitates as solid crystallites. In polycrystalline form, the crystallites are small (microscopic) and do not form a perfect interface, rather a discontinuity forms between crystals. The transition between solid crystallites of the alloy is known as the grain boundary. Grain boundaries can exist throughout a formed alloy. After formation, certain alloys that are exposed to heat undergo what is termed “sensitization”.
Exposure to environmental heat of metastable alloys causes specific atoms in that alloy to migrate to grain boundaries in the alloy to form a stable phase. The migration of atoms to grain boundaries eventually causes the grain boundaries to have different physical and chemical characteristics from the portion of the alloy not located at grain boundaries. Sensitization is the migration of a specific kind of atoms to grain boundaries in an alloy where a new phase (crystalline structure) is formed, different than what may have been intended for the homogeneous alloy. If the phase is corrosive, sensitization in an alloy causes it to become susceptible to intergranular corrosion.
Intergranular corrosion is a form of corrosion that affects the grain boundaries of an alloy. If a sufficient amount of intergranular corrosion occurs, then the alloy may undergo more detrimental forms of corrosion, for example, exfoliation corrosion and stress-corrosion cracking. Exfoliation corrosion is a form of corrosion that causes the complete separation of layers of the alloy. Stress-corrosion cracking is a form of corrosion that causes an incipient crack in the alloy to grow under mechanical stress. Exfoliation corrosion and stress-corrosion cracking are advanced forms of intergranular corrosion which may lead to the structural failure of the alloy. Structural failure resulting from intergranular corrosion may be avoided by early detection of sensitization.
Methods have been developed to measure sensitization for various alloys based on the change in chemical characteristics of the alloys. For example, sensitization of steel may be tested using an oxalic acid test, a ferric sulfate-sulfuric acid test, a nitric acid test, and copper-copper sulfate-50% sulfuric acid test. For aluminum alloys, sensitization may be tested by immersion tests in sodium chloride hydrogen peroxide solution, and by a nitric acid mass lost test. Generally, these tests may be difficult to use, are time consuming, they destroy the alloy tested, and require the use of dangerous chemicals.
Alternatively, ultrasonic testing can measure sensitization of an alloy nondestructively (not affecting the test piece) based on the change in physical characteristics of the alloy. Ultrasonic testing is generally nondestructive, and may be safer, easier to use, and quicker than tests based on the change in chemical characteristics. Determining sensitization using ultrasonic testing is done by transmitting ultrasonic waves through the alloy and measuring the velocity and attenuation of the received waves. The two types of waves measured are compressional-waves and shear-waves. A change to the physical characteristics of an alloy can cause ultrasonic waves to scatter and/or be absorbed differently, as well as travel with different velocities. As the atomic bonds change with the structure, the elastic moduli of the alloy change, and, with them, the sound velocities change. The combined effect of scattering and absorption is called attenuation. The velocity and attenuation of the propagating waves are dependent on the degree of physical change to the alloy and are consequently valuable ultrasonic parameters or determining sensitization.
Ultrasonic testing may be performed with the use of various devices including ultrasonic transducers. The configurations of these devices can vary, but generally fall into two basic categories called active transducers and passive transducers. An active transducer is a device that converts alternating electrical field into mechanical waves, and in reverse, converts mechanical waves into alternating electrical current. Passive transducers only convert incoming mechanical waves into alternating electrical current. Both active transducers and passive transducers can be made of piezoelectric crystals which generate voltage when they change shape by being stressed mechanically. Ultrasonic testing can be performed in various configurations with ultrasonic transducers in a variety of measuring methods. The combination between ultrasonic devices used and measuring methods is called an ultrasonic measuring technique.
One example of an ultrasonic measuring technique is a pulse-echo technique (“PE”). PE uses an active transducer that is coupled to the surface of the alloy tested, The transducer transmits pulses which pass through the alloy and are reflected by interfaces (such as solid-gas or solid-liquid). The returned ultrasonic pulses are converted into electrical signals by the same transducer working in reverse mode.
Another example of an ultrasonic measuring technique is a resonant ultrasound spectroscopy technique (“RUS”), that acts like a biopsy test. With the RUS, a small amount of alloy (e.g., 8 millimeters cube) is mechanically coupled between two piezoelectric transducers. The first transducer generates ultrasonic waves directed at the alloy. The second transducer is a passive transducer and detects the sample alloy's resonance.
In another ultrasonic measuring technique, an electromagnetic acoustic transducer technique (“EMAT”) is used to test sensitization ultrasonically. EMAT uses a transmitted pulse and the receipt of a reflected pulse similar to PE. However, the active transducer used in EMAT is not mechanically coupled to the alloy.
Because the methods currently used to measure sensitization of alloys based on chemical reactions are difficult to use, time consuming, destroy the alloy tested, and require the use of dangerous chemicals, and because the methods may be unreliable and have errors associated with testing, there is a demand for a safer, easier-to-use, nondestructive, and accurate test for determining the sensitization of alloys.
The present invention satisfies the demand by providing a safer and easier-to-use test capable of accurately determining the sensitization of alloys without destroying the tested alloys.